Touch panels constitute an independent class of devices. The devices include a panel, usually of a rectangular shape, whose function is to determine coordinates of the touch site relative to the screen boundaries in an analog or digital form. The screen can be touched by a finger, or a special stick (stylus), or another mediator, depending on the panel type. Some devices perform additional functions of determining the size and shape of the touch area and/or the pressing force.
In liquid crystal displays, touch panels serve as additional components making it possible to use the display area for both output and input of data, which can be employed in various fields. In this case, the touch panel, besides being sensitive to pressure, must be transparent. This is most convenient for the user and allows the data (command) to be introduced simply by touching an image on the display. With the screen of a computer, communicator, mobile telephone, etc., functioning in this mode, it is possible to introduce data by writing directly on the screen, or to realize a control menu or a keyboard by imaging these elements on the screen, etc. In such cases, the most adequate term for a device combining the functions of a transparent touch panel and a liquid crystal display is a touch screen or touch-sensitive screen.
Presently, there are five principal technical solutions, which provide for the possibility to create transparent touch panels for use in touch-screen systems.    (a) Resistive panels. In panels of this type, the surface sensitivity to pressure is provided by using a thin conducting elastic layer separated from another conducting layer by insulating spacers. Touching the panel brings the two conducting layers into contact, after which the touch is detected by a change in the applied voltage.    (b) Capacitive panels. These implement a thin conducting layer, to which a constant voltage is applied, and an external insulating film. Touching the panel gives rise to an induction current in the thin conducting layer, which is used to detect the touch.    (c) Near-field imaging panels. Touch panels based on this principle contain a conducting layer of special internal structure. Application of a constant voltage to this layer gives rise to an electric field near the panel surface. A finger or stylus approaching the panel surface introduces distortion into this field, by which location of the touch site is determined.    (d) Surface acoustic wave panels. In this case, a source of ultrasound (piezoelectric cell) generates a stationary acoustic (ultrasonic) field in a glass panel. A finger or stylus touching the surface absorbs ultrasound and, hence, modifies this field. This change is detected by ultrasonic sensors (piezoelectric sensor) whose positions determine the touch site.    (e) Infrared panels. These panels employ pairs of linear arrays of point IR radiation sources and sensors arranged opposite to each other along the boundaries of the screen, close to its surface. Each array of sources illuminates the opposite array of sensors, one pair being situated on the horizontal boundaries and the other, on the vertical boundaries. Any object introduced into the near-surface region shadows the corresponding zone in the array of sensors, thus locating the touch site.
Liquid crystal displays (LCDs) are among the most widely used devices for the display of graphic, alphanumeric, symbolic, and other kinds of information [S.-T. Wu and D.-K. Yang, “Reflective Liquid Crystal Displays”, Wiley (2001); E. Lueder, “Liquid Crystal Displays: Addressing Schemes and Electro-Optical Effects”, Wiley (2001)].
In LCDs, the light from a front or backlighting system or from ambient light sources passes sequentially through the plurality of functional layers. The functionality of the display requires at least a polarizer, a liquid crystal (LC) layer confined between transparent electrodes, and transparent substrates of glass or plastic to be included in this stack. Also the required elements typically include alignment layers providing the orientation of the directors of the liquid crystal molecules at the boundaries of the liquid crystal layer. The transparent substrates are required to protect the liquid crystal and other layers confined between them from mechanical and other damage. In the reflective LCDs there is also a reflective layer at the rear of the optical stack. Additionally the plurality of functional layers could include a second polarizer, retardation plates, color filters, planarization and protective layers, insulating layers and other layers depending on the technical requirements of the display and its functions.
In twisted-nematic (TN) type of LCD which operate in the normally-white (NW) mode, the capability to modulate the intensity of the light passing through the functional layers of the display is realized with the liquid crystal confined between transparent electrodes, which are in turn confined between the couple of the polarizers with crossed transmission axes. If a voltage is applied to the LC layer with the aid of electrodes completely suppresses the twist effect, the polarization of light created by the first polarizer remains unchanged and the light is absorbed in the second polarizer oriented perpendicularly to the first one (crossed polarizers). On the contrary, when no voltage is applied to the LC, the polarization plane of the light is rotated so that the beam passes the second polarizer without absorption.
The above scheme can exhibit significant variations depending on features of the LCD design. There are two main variants of LCDs: reflective and transmissive. Displays of the first type use the light from ambient sources and employ no special backlighting systems, thus consuming a minimum of power. Transmissive displays are provided with backlighting systems employing light sources situated on the side opposite of the display to that viewed by an observer. A reflective display with semi-transparent mirror and a backlighting system behind it can operate in both reflection and transmission modes. LCDs of this hybrid type are called transmissive-reflective (transflective).
In describing LCDs, it is convenient to differentiate between front and rear sides. The front side is that facing the observer as well as the front lighting system of ambient sources, the rear side is opposite to the front side. A set of layers in the LCD structure situated in front of the LC layer is frequently referred to as the front panel, while layers behind the LC layer are called the rear panel. Accordingly, the like functional layers situated in these panels are specified as “rear” and “front”; for example, there are rear and front substrates, rear and front electrodes, etc. It is also possible to specify opposite sides of any layer in a given LCD.
In the conventional LCD design with a touch panel (FIG. 1), the transparent touch panel 1, secured to the additional front transparent substrate 2, is situated in front of the LCD proper. The additional transparent substrate protects the front polarizer 3 from mechanical damage when the touch panel is secured to the display. The front transparent substrate 4 is followed by a transparent ITO electrode 5, front alignment film 6, and LC layer 7. Situated behind the LC layer in reverse order are the rear alignment film 6, transparent ITO electrode 5, transparent substrate 4, and rear polarizer 8. The LCD structure may also include a reflecting layer (9), retarders, air gaps, insulating films, planarization layers, protective films, etc.
An important characteristic of any LCD is the twist angle of the director of LC molecules, that is, the angle by which the director rotates on the passage from one to another side of the crystal. If this angle falls within the interval from 180° to 300°, the LC has a small switching voltage and the LCD transition from transparent to nontransparent state takes place at a small variation of voltage applied to the electrodes. LCDs of this type are called supertwisted nematic (STN) displays and are employed in systems featuring passive matrix addressing with a relatively high number of addressable pixels. STN displays are said to support a high level of multiplexing of applied voltages.
The LCD operation scheme significantly changes when the LC birefringence value (determined by a difference between the optical path lengths of the ordinary and extraordinary rays) becomes close to a light wavelength in the visible spectral range (400–700 nm). In this case, a simple conception about rotation of the light polarization plane upon going through the LC is no longer adequate to the real process. It was established [C. H. Gooch and H. A. Tarry, The Optical Properties of Twisted Nematic Liquid crystal Structures with Twist Angles Below 90°, Journal of Physics D, 8, 1575 (1975)] that a light passing through a thin liquid crystal layer of this type exhibits a change in the polarization from linear to elliptic. In the general case, an LCD based on such a liquid crystal layer will always (i.e., for any state of the LC) partly transmit the light through both crossed polarizers and, hence, the contrast of the image on display will drop. In order to suppress this effect and increase the contrast of LCDs with very thin liquid crystal layers, special mixed modes of the liquid crystal operation were developed.
On the other hand, using thin liquid crystal layers allows more achromatic image to be obtained because a decrease in the LC layer thickness results in weaker dispersion effects, which improves the color rendering. In addition, thin liquid crystal layers typically provide better viewing angle performance. However, since LCDs employing thin liquid crystal layers operate under more constrained conditions, such systems must take into account the influence of all LCD layers on the image quality.
All transparent touch screens contain a layer of glass or transparent plastic that serves as a base for the panel and protects its elements from the action of mechanical factors. Resistive panels contain two thin conducting layers separated by an air gap determined by insulating spacers. Capacitive panels contain at least one thin conducting layer, sometimes of a complicated topology. In the touch panels employing surface acoustic waves, ultrasonic oscillations in the near-surface layer of glass may give rise to local oscillations in the optical density of glass. Any modification of the touch panel design brings additional layers (with nonzero absorption in the visible spectral range, refractive index mismatch with surrounding optical layers, and other significant optical characteristics) and introduces additional interfaces. For example, an increase in the number of layers in the LCDs with resistive panels may lead to a 75% loss of the throughput luminance. In addition, increased reflection from the additional optical interfaces can dramatically reduce the contrast of the display, particularly where large refractive index differences exist at interfaces such as those created by air gaps in the optical stack. Therefore, it is necessary to take into account the influence of a transparent touch panel on the luminance, contrast, achromatism, color rendering, and angular properties of LCDs.
The present invention focuses on the possibility to improve the quality of LCDs with an integrated touch screen, and particularly to increase the luminance, contrast, and mechanical stability of LCDs with an integrated touch screen, by using coatable thin-film polarizers on the inside of the liquid crystal cell instead of the relatively thick multi-layer dichroic sheet polarizers which must typically be mounted external to the liquid crystal cell. There is a known LCD with internal polarizers (see U.S. Pat. No. 6,399,166 B1). A special feature of this design is that the internal polarizers are made in the form of coatings on the inner surfaces of transparent substrates.
Below are listed the known prior art for touch screen LCDs, selected by us in the capacity of analogs.
There is a known LCD with touch-sensitive screen (GB 2367991), representing a display combined with a special system for detecting a user's touch on the screen and an additional system for switching the image on screen upon this touch.
There is a known LCD with touch-sensitive screen (U.S. 2001-0020578), in which the functions of display and touch screen are combined by using the LCD electrodes as contact plates. The electrodes are brought into contact by pressing on the display front surface; alternatively, they may serve as electrodes of an induction sensor, finger or stylus being the other electrode. It was also suggested to equip the device with an optical (infrared) pressure sensor, which operates when the radiation is reflected from finger or stylus. Disadvantages of this system are (i) the possible distortion of the image on the display caused by the LC deformation under pressure and (ii) complication of the LCD design and technology as a result of combining the display and touch screen functions.
There is a known LCD with touch-sensitive screen (WO 0127868), in which the pressure-sensitive functional layer represents a transparent conducting plate to which a digital electronic scheme is connected that monitors a change in the electrode capacitance when a conducting objects approaches the screen surface. A drawback of this solution is that a special stylus is required.
There is a known LCD with touch-sensitive screen (GB 2,344,905), in which the touch panel and display screen are situated on the opposite sides of a compact manual device. A disadvantage of this solution is the difficulty of implementing such a design in large-size systems.
There is a known LCD with touch-sensitive screen (WO 9953432), in which the pressure-sensitive panel includes a polarizing or scattering film which is schematically and technologically separated from the front and rear panels. Disadvantages of this solution are insufficient luminance and low contrast.
There is a known LCD with touch-sensitive screen (WO 0157841), in which a polarizing layer is confined between an external substrate and the front electrode. Drawbacks of this design are complications in the fabrication technology (caused by combining the polarizer and touch panel functions in the same element) and a decrease in the display luminance and contrast.
There is a known LCD of the reflective type with touch-sensitive screen (U.S. Pat. No. 5,105,186), in which the mirror is semitransparent and a photosensitive matrix is placed behind it to determine the position of a shadow from an object touching the front surface. Disadvantages of this solution are (i) a decrease in luminance and contrast caused by the losses of light flux from the ambient sources in the matrix and (ii) dependence of the touch screen sensitivity on the intensity of ambient sources. The use of a photosensitive matrix, which is an expensive element, may significantly increase the cost of such LCD.